1. Field of the Invention
The present invention relates to a booster type converter.
2. Description of the Related Art
Heretofore, a booster type converter by which a DC output having a prescribed voltage value higher than that input from a DC power source can be supplied to a load by controlling a switching duty of a boosting switching element in the booster type converter has been widely used as a power unit for electronic and electrical equipments.
FIG. 1A is a block diagram showing an example of a conventional booster type converter 1. This booster type converter 1 comprises a DC power source 3, a + output terminal 3a of the DC power source 3, a - output terminal 3b of the DC power source 3, a reactor coil unit 5, a power MOSFET element 7, a main diode element 9, a smoothing capacitor 11, a + output terminal 13a, a - output terminal 13b, and a control circuit 14.
The booster type converter 1 is constituted in such that the + output terminal 3a of the DC power source 3 is connected to an end of the reactor coil unit 5, a drain side of the power MOSFET element 7 is connected to the other end of the reactor coil unit 5, a source side of the power MOSFET element 7 is connected to the - output terminal 3b of the DC power source 3, a connecting midpoint between the drain side of the power - MOSFET element 7 and the other end of the reactor coil unit 5 is connected to an anode side of the main diode element 9, a cathode side of the main diode element 9 is connected to the + output terminal 13a, a + pole side of the smoothing capacitor 11 and a control signal input end 14a of the control circuit 14 are connected to a connecting midpoint between the cathode side of the main diode element 9 and the + output terminal 13a, a switching signal output end 14b of the control circuit 14 is connected to a gate side of the power MOSFET element 7, the source side of the power MOSFET element 7 is connected to a - pole side of the smoothing capacitor 11, and this contact point is connected to the - output terminal 13b, respectively.
In the above described constitution, it is intended to arrange that a voltage produced in the reactor coil unit 5 is cumulated to a voltage value of the DC power source 3, and the resulting voltage is accumulated in the smoothing capacitor 11 through the main diode element 9 for discharging the energy during an OFF period of a switching signal which energy is accumulated in the reactor coil unit 5 during an ON period of the switching signal supplied to the gate side of the power MOSFET element 7 by controlling respective ratios of time width in respect of the OFF period and the ON period in the switching signal supplied to the gate side of the power MOSFET element 7 in the control circuit 14 based on an output voltage value of the +output terminal 13a in such that an output voltage value defined between the + output terminal 13a and the - output terminal 13b is kept constant at a predetermined value, whereby a voltage having a constant voltage value which is obtained by elevating a voltage value of the DC power source 3 can be applied to a load from the +output terminal 13a and the - output terminal 13b.
It is to be noted that a period designated by "OFF" represents such a period wherein a switching signal supplied to the gate of the power MOSFET element 7 from the switching signal output end 14b of the control circuit 14 is in an OFF state, while a period designated by "ON" represents such a period wherein the switching signal is in an ON state, respectively, in FIG. 1B.
In the booster type converter 1 shown in FIG. 1A, because of production of an overlapped area designated by VIW of a voltage VDS existing between the drain and the source of the power MOSFET element 7 and a drain current ID as shown in FIG. 1B at the time when the switching signal supplied from the switching signal output terminal 14b of the control circuit 14 to the gate of the power MOSFET element 7 is turned from an "ON" state to an "OFF" state as shown in a waveform of FIG. 1B, a switching loss is generated at the power MOSFET element 7 in the overlapped area VIW, and heat is generated in the element 7 due to electric power loss derived from the above described loss, thereby to decrease a DC--DC conversion efficiency of the booster type converter 1.
Since an amount of the heat generation increases in proportional to a switching frequency value of the element 7, there has been such a disadvantage in that an improvement in the switching frequency value is limited in view of restrictions of prevention in thermal breakdown of the power MOSFET element 7.